This invention relates to a thrust preventive device for the output shaft of an electric motor wherein the output shaft is held in place, being kept biased in one direction taken along its central axis, by attracting or repelling a rotating force transmitting member fixed to it by the action of a magnetic force generated in a magnetic force generating portion provided at an end face of the motor from which is projected the output shaft.
An electric motor, particularly a coreless motor for use in a tape recorder for example, is required to rotate always at a fixed speed. To this end, it is necessary to keep always at a fixed value the friction loss of the parts associated with the motor rotation. Therefore, the load which is applied to the portions of sliding contact between a brush and a commutator of the motor or between the output shaft and a journal therefore must be kept fixed at all times throughout the motor rotation.
Meanwhile, where the tape recorder is of a portable or a pocketable type which can be put into the pocket of men's apparel or in a suit pocket, such tape recorder during its operation is not used being set at a fixed position but is usually used while it is being inclined at various angles taken with respect to the vertical or horizontal level. In this case, the output shaft of the motor, due to the weight of the relevant rotor, is arbitrarily moved in one direction or in the opposite along the central axis of the shaft. This causes a change in the magnitude of a load applied to the journal or in the direction in which such load is applied. This causes a change in the rotating speed of the motor, which change may give rise to wow and flutter.
A conventional coreless motor, therefore, for the purpose of eliminating such drawbacks, is designed such that the rotating force transmitting member comprised of a fly-wheel portion formed of a material magnetizable by a magnetic line of force and a rotating force transmitting portion constituted, for example, by a pulley is secured, by means of, for example, an adhesive or close fit, to the output shaft of the motor, whereby said fly-wheel portion is attracted by the magnetic line of force of a permanent magnet provided inside the motor to rotate the output shaft. In such a conventional coreless motor, however, once the fixation of the rotating force transmitting member to the output shaft is completed, it is difficult to change the position of the member relative to the output shaft. After such fixation, therefore, it becomes impossible to finely control the magnitude of that thrust of the output shaft which is produced by the attraction of the fly-wheel portion by the magnetic line of force.